1.3 Chemical calculations

Question 1

What does the term relative atomic mass mean?

Answer 1

The average mass of one atom of an element compared to 1/12th the mass of an atom of carbon-12.

Question 2

What does the term relative formula mass mean?

Answer 2

The weighted average of the masses of the formula units compared to 1/12th the mass of an atom of carbon-12.

Question 3

What does the term relative isotopic mass mean?

Answer 3

The mass of one atom of an isotope compared to 1/12th the mass of an atom of carbon-12.

Question 4

What is mass spectrometry?

Answer 4

Mass spectrometry is a type of analytical technique which measures the mass to charge ratio of ions. It also measures how abundant each ion is so it can be used to calculate the relative atomic mass.

Question 5

Explain the five stages of TOF mass spectrometry

Answer 5

1. lonisation - vapourised sample is injected into the mass spectrometer where it is ionised.
2. Acceleration - the positively charged ions are accelerated towards a negatively charged detection plate.
3. lon drift - the ions are deflected into a curved path with a magnetic field.
4. Detection - the positive ions are detected by hitting a negatively charged plate, where they gain an electron, producing a current. The greater the current, the greater the abundance.
5. Analysis - the relative abundance of each isotope are compared. Relevant calculations are made.

Question 6

How can you use a mass spectrum to deduce the relative molecular mass of a sample of a compound?

Answer 6

The peak with the highest m/z value (the molecular ion peak, M⁺) is caused by the whole molecule, therefore m/z value = molecular mass.

Question 7

Define empirical formula

Answer 7

The simplest whole number ratio of atoms of each element in a compound.

Question 8

Define molecular formula

Answer 8

The actual number of atoms of each element present in a compound.

Question 9

What is the empirical formula for Fe₂O₄?

Answer 9

FeO₂

Question 10

What does the term relative molecular mass mean?

Answer 10

The weighted average of the masses of the molecules compared to 1/12th the mass of an atom of carbon-12.

Question 11

A molecule has the empirical formula C₄H₃O₂ and a relative molecular mass of 166. What is the molecular formula?

Answer 11

Empirical mass = 4(12) + 3(1) + 2(16) = 83

Relative molecular mass = 166

Compare molecular mass with empirical mass = 166 / 83 = 2

So the molecular formula is double the empirical formula: C₈H₆O₂

Question 12

Define the Avogadro constant

Answer 12

The number of particles in one mole of a substance. This is 6.02 x 10²³ particles.

Question 13

What is the equation linking Avogadro’s constant to moles?

Answer 13

Number of particles =
Moles x Avogadro’s constant

Question 14

There are 4.816 x 10²⁴ atoms of iron in a sample.
How many moles of iron are in the sample?

Answer 14

Avogadro’s constant = 6.02 × 10²³ mol⁻¹
Moles = number of atoms ÷ Avogadro’s constant
= (4.816 × 10²⁴) ÷ (6.02 × 10²³)
= 8 mol of Fe

Question 15

Define molar mass

Answer 15

The mass per mole of a substance, measured in g mol⁻¹.

Question 16

What is the equation that links moles to mass?

Answer 16

Moles =
Mass (g) x Relative atomic mass

Question 17

Define concentration

Answer 17

The amount of moles per unit volume.

Question 18

How can concentration be calculated in g/dm³?

Answer 18

Concentration (g/dm³) =
Mass (g) / Volume (dm³)

Question 19

How can concentration be calculated in mol/dm³?

Answer 19

Concentration (mol/dm³) =
Moles / Volume (dm³)

Question 20

5.00 g of NaCl is dissolved in 25 cm³ of water.
Calculate the concentration of the solution in mol/dm³.

Answer 20

Moles of NaCl = 5 / 58.5 = 0.0855
Volume in dm³ = 25 / 1000 = 0.025
Concentration in mol/dm ³ = 0.0855 / 0.025 = 3.42 mol/dm³

Question 21

What is the molar volume of any gas at room temperature and pressure?

Answer 21

24 dm³

Question 22

What is RTP?

Answer 22

Room temperature and pressure:
- 20°C
- 1 atmosphere

Question 23

What equation links molar volume at RTP to moles?

Answer 23

Volume of gas at RTP (dm³) = moles x 24

Question 24

How many moles of oxygen are in 72 dm³ at RTP?

Answer 24

Moles = volume / 24
= 72 / 24
= 3 moles

Question 25

How is the molar volume at a given pressure affected if temperature is increased?

Answer 25

An increase in temperature reduces the molar volume at a given temperature.

Question 26

How is the molar volume at a given temperature affected if pressure is increased?

Answer 26

An increase in pressure will mean the maximum molar volume reduces. Up to this maximum, increasing the pressure will increase the molar volume.

Question 27

What is the ideal gas law?

Answer 27

pV = nRT
p - pressure (Pa)
V - volume (m³)
n - number of moles
R - gas constant (8.31 J K⁻¹mol⁻¹)
T - temperature (K)

Question 28

At a temperature of 55.0 °C and a pressure of 275 kPa, a gas occupies a volume of 1.10 dm³. How many moles of the gas are present? The gas constant is 8.31 J K⁻¹ mol⁻¹.

Answer 28

pV = nRT
- 275 kPa is 275000 Pa
- 55.0 °C is 328 K
- 1.10 dm³ is 0.001 m³

Therefore, n=(pV)(RT)
n=(275000 x 0.001)/(8.31 x 328)
n= 0.10 moles

Question 29

How can percentage yield be calculated?

Answer 29

Percentage yield =
(Actual yield ÷ Theoretical yield) × 100

Question 30

What is the percentage yield of NH₃, if 40.5 g of NH₃ is produced from 20.0 mol H₂ and excess N₂?

Answer 30

Equation: N₂ + 3H₂ → 2NH₃
Moles of ammonia = 20/1.5 = 13.3 moles
Mass of ammonia = 13.3 x (14+1+1+1) = 227 g
Percentage yield = (40.5/227) × 100 = 17.9%

Question 31

Why might the actual yield of product be less than expected?

Answer 31

• Incomplete reaction
• Unwanted side reactions
• Practical losses, for example some solid may get lost when being transferred between beakers

Question 32

What is the atom economy of a reaction?

Answer 32

Atom economy is a measure of the efficiency of the reaction. It looks at the amount of reactants that get turned into useful products.

Question 33

How can atom economy be calculated?

Answer 33

Atom economy=
(Mr of desired product / total Mr of reactants) × 100

Question 34

Fill in the gap: ‘The _______ the atom economy, the more sustainable and efficient the process’

Answer 34

Higher

Question 35

How can percentage error be calculated?

Answer 35

% error = absolute uncertainty/ calculated value x 100

Question 36

A titre volume is recorded as 11.30 cm³. The accuracy of the burette is ±0.05 cm³. Calculate the maximum percentage error.

Answer 36

% error = absolute uncertainty/ calculated value x 100
(0.05 ÷ 11.30) × 100 = 0.442%

Question 37

What is meant by stoichiometry?

Answer 37

Stoichiometry is the use of balanced chemical equations to calculate the amount of products and reactants used in the reaction. This mainly uses ratios between the species of the reaction to calculate quantities, such as reacting masses.